Generally, nitrogen oxides are generated from stationary sources such as e.g. industrial boilers, gas turbines, steam power plants, waste incinerators, marine engines, and petrochemical plants. The selective catalytic reduction (SCR) is considered a useful approach for removing nitrogen oxides generated from stationary sources in view of economic and technological efficiency. A wide number of catalysts have been reported for the effective removal of nitric oxide by using ammonia as the reducing agent. All the catalysts can broadly be classified into three types namely noble metals, metal oxides and zeolites. Noble metals are very active for the reduction of NOx, but do not reduce selectively to N2 because of ammonia oxidation. Side products like N2O might also be formed. Accordingly, noble metal catalysts have been replaced by metal oxide catalysts for conventional SCR and by zeolites for high temperature SCR applications because of their thermal stability.
SCR may thus be deemed a well-proven technology as regards its application with conventional, non-renewable fuels. However, over the past two decades there has been an increasing interest globally in the utilization of non-conventional fuels like biomass for energy production. Biomass such as wood and straw are CO2 neutral fuels which may help to reduce the greenhouse effect. According to the latest official estimate, Denmark has approximately 165 PJ (petajoule) of residual biomass resources including waste, of which only half are currently used. Residual resources comprise straw, which is not needed for animal purposes, together with biogas from manure, organic waste and waste from wood industries. However, the potential of biomass fuels from a change of crops is huge. Denmark grows a lot of wheat which can be replaced by other crops such as corn, leading to a much higher biomass production while still maintaining the same output for food. Such reorganisation of the farming areas together with a few other options may lead to a total biomass fuel potential as high as 400 PJ.
In the EU, so far two binding directives have been enacted which set quantitative targets for renewable energies and fuels in the current and future energy supply up to 2010. In Directive 2001/77/EC on the promotion of electricity produced from renewable energy sources in the internal electricity market (2001) and Directive 2003/30/EC on the promotion of the use of biofuels or other renewable fuels for transport (2003), the target for renewable electricity is set to 22%, the target for biofuels set to 5.75%, and the target for total renewable energy consumption is set to 12%. Until 2020, these targets are to be enlarged considerably according to EU Renewable Energy Road Map—Renewable energies in the 21st century: building a more sustainable future (2007). Given that nearly 66% of renewable energy production in the EU in 2004 was based on biomass (hereafter referred to as bioenergy), the demand for biomass will increase rapidly during this time horizon.
The same trend is observed in the US, where biomass sources provide a small, but growing percentage of all energy consumed. In 2002, biomass supplied about 47 percent of all renewable energy consumed in the United States. Electricity generation from biomass (excluding municipal solid waste) represented about 11 percent of all generation from renewable sources in the United States. In fact, biomass supplied more energy to the US in 2002 than any other form of renewable energy, including hydroelectric power. Biomass supplied almost six times the energy of geothermal, solar and wind energy sources combined. Globally, biomass meets about 14 percent of the world's energy needs.
Thus, the worldwide use of biomass for production of energy is expected to keep an ascendant trend despite of its rather low caloric value.
The main pollutants resulting from biofuels are nitrogen, chlorine, potassium and silicon, the main emission being NOx, which may be reduced significantly by applying SCR technology. However, even though SCR is a well-proven technology, its application with non-conventional fuels like biomass brings about specific challenges. In particular, deactivation of the catalyst by biomass containing alkali metals and subsequent activity reduction is problematic. Flue gases stemming from the incineration of biomass fuel typically contain about 200-1000 mg KCl/Nm3 whereas incineration of coal only leads to ppm levels of KCl.
Heteropoly acids (HPAs) and salts thereof are a class of compounds that have attracted much scientific interest. Because of their unique structure and the resulting acidic and redox properties, they have been studies as possible catalysts for a variety of reactions. HPAs possess unique physicochemical properties, with their structural mobility and multifunctionality being the most important for catalysis. Consequently, acid catalysis and selective oxidation are the major areas of catalytic applications of HPAs.
The class of HPAs can in broad general terms be described a compound containing 1) an addenda metal such as tungsten, molybdenum or vanadium, 2) oxygen, 3) a hetero atom being an element generally from the p-block of the periodic table, such as silicon, phosphorus or arsenic, and 4) acidic hydrogen atoms. The hetero atom(s) are situated in the center of the HPA structure with clusters formed by the addenda metals and the oxygen atoms situated around the centrally placed hetero atom(s).
The best known structural groups of HPAs is the Keggin structure (HnXM12O40) and the Dawson structure (HnX2M18O62), wherein M denotes the addenda atoms and X is the hetero atom(s). The Keggin and Dawson structures exist in different isomers and may contain more than one type of metal addenda atoms. Thus, there exist in a large variety of possible HPAs. An example of Keggin and Dawson structure are shown in FIG. 14.
The majority of catalytic applications use the most stable and easily available Keggin HPAs, especially for acid catalysis. Most typical Keggin HPAs such as H3PW12O40 (TPA), H4SiW12O40 (TSiA) and H3PMo12O40 (MPA) are commercially available. HPAs possess stronger (Brønsted) acidity than conventional solid acid catalysts such as acidic oxides and zeolites. The acid strength of Keggin HPAs decreases in the order: H3PW12O40>H4SiW12O40>H3PMo12O40>H4SiMo12O40. The acid sites in HPA are more uniform and easier to control than those in other solid acid catalysts. Usually, tungsten containing HPAs are the catalysts of choice because of their stronger acidity, higher thermal stability and lower oxidation potential compared to molybdenum acids.
It has previously been found that the 12-tungstophosphoric acid H3PW12O40 (TPA) can effectively absorb NO at the flue gas temperatures, and that upon rapid heating, the absorbed NO is effectively decomposed into N2. The results showed that the quantity of NO2 retained on TPA is strongly dependent on temperature: increasing from 298 K reaches a maximum in the range from 423 to 573 K, and decreases to small values from 773 to 873 K. The results further showed that the quantities of NO2 lost from the gas phase follow the order H3PW12O40>H4SiW12O40>H3PMo12O40. Supplementary experiments showed that the maximum quantity of NO taken up by the solid is approximately equal to those of NO2. The adsorption of NO occurs via replacement with the structural water present between the Keggin units in heteropoly acids. NOx adsorption/desorption capacities of TPA were measured under representative exhaust lean gas mixture conditions with a real car exhaust mixture containing, for example, CO2, H2O and hydrocarbons. The results proposed a mechanism of both NOx absorption and desorption on TPA.
Later Pt/TPA and TPA supported metal oxides were also used extensively for the abatement of NOx majorly, for the mobile applications. Recently, Pd was loaded on the dispersed H3PW12O40 (TPA) over a SiO2 surface, and the catalyst was applied to the selective reduction of NO with aromatic hydrocarbons for the stationary applications. The catalyst exhibited high activity in the NO reduction when branched aromatic hydrocarbons, such as toluene and xylene were used as reductants.
The deactivation effect of alkaline metals on the activity of V2O5/TiO2 catalysts for the biomass fired applications in power plants has been well reported in the literature. Most of these reports conclude that poisonous additives (e.g. potassium, barium) are affecting the Brønsted acid sites, which are responsible for the ammonia adsorption, thus decreasing both their number and activity in NO reduction. One of the possible ways to increase catalyst resistance to alkaline poisons is the use of supports, revealing high or super-acidic properties which would interact stronger with alkali than vanadium species. One such super-acidic characteristics are available in heteropoly acids also.
Heteropoly acids are typical strong Brønsted acids and catalyze a wide variety of reactions in both homogeneous and heterogeneous phases offering efficient and cleaner processes. For practical applications, it is important to improve the physical properties of HPA, e.g. by improving the mechanic and thermal resistance. This could be reached by depositing HPA on a suitable support while preserving its chemical properties (absorption capacity). Dispersing HPA on solid supports is important for catalytic application because the specific surface of unsupported HPA are usually low, although interstitial voids are created by the terminal oxygen atoms linking the hydrated protons because these are not interconnected the resulting solid acid have low BET (N2) surface areas 1-10 m2 g−1.
In general, HPA strongly interact with supports at low loading levels, while the bulk properties of HPA prevail at high loading levels. To overcome these disadvantages the HPA are usually supported on a suitable carrier that not only increases the available surface area but also improves the catalytic performance. The selection of proper support material has to take into account the strong acidity of HPAs. If a support is moderate to strongly basic (e.g., Al2O3, MgO), the interaction with HPA is too strong and leads to an acid-base reaction with loss of crystallinity of HPA with a complete degradation of its storage properties. If the support is strongly acidic (e.g., SiO2), X-ray diffraction (XRD) structure of HPA exists, but the anchorage is not secured. In the case of medium acidity (e.g., ZrO2, TiO2 and SnO2), the structural properties are retained and the activity remains high. Consequently, oxides supports can be selected from their isoelectric point (around 7).
To the best of our knowledge, the use of HPA's as a promoter in the selective catalytic reduction of NOx in exhaust or flue gases obtained from burning biomass is not disclosed anywhere in the literature. Also, the problem of alkali metals being present in exhaust gases released on burning biomass, which will normally lead to fast and irreversible poisoning of standard commercial SCR deNOx catalysts it not discussed in the literature.
There is consequently still a need for developing SCR catalysts which may function well under the specific and very demanding conditions of biomass incineration, and at the same time be sufficiently robust to allow for uninterrupted performance over long time periods.